This invention relates to ultra-miniature electrical contacts which are small enough and robust enough to make contact directly and reliably to micron size areas at any of numerous contact points on the face of an integrated circuit (IC), for example.
As integrated circuits (ICs) become denser and denser with thousands and even millions of devices (e.g., memory cells, gates, etc.) on a single chip, the problem of making connections between external circuits and the devices on the chip becomes increasingly difficult. Frequently, a chip is encapsulated in a package with contacts on the chip permanently connected (e.g., by wire bonding, or otherwise), to relatively large external contacts on the package, such as the pins on a dual in-line package (DIP). Especially where the devices on an IC operate at speeds in the gigahertz (GHz) range, this method of connecting the internal devices on the chip to external circuits can cause impedance mismatching and imposes undesirable restraints on the overall speed of operation as well as significant added costs. It would therefore be highly desirable to be able to make connections directly to the devices on the chip with minimal physical separation from other devices, or from one chip to another.
Another widely used way to make connections to an IC is by means of solder xe2x80x9cbumpsxe2x80x9d positioned in a pattern either on a substrate that matches the locations of the solder bumps to respective locations of small surface pads on the IC, or on the IC itself. The solder bumps are pressed against the pads and then reflowed thereby making permanent interconnections all at once. However, one disadvantage with this way of interconnection is that it results in a very rigid composite structure. The IC and substrate are held closely together with perhaps a thousand soldered joints. Unless the coefficients of thermal expansion of the IC and of the substrate are closely matched, differential expansion of the members when temperature changes during operation can cause rupture of an interconnection or fracture of the IC or substrate. It is highly desirable therefore that interconnections between an IC and a substrate be at least somewhat mechanically compliant. The present invention provides interconnections which while stable and low in resistance are also compliant.
From a cost standpoint, it would also be highly desirable in order to meet any particular or specific application to be able to assemble into an overall circuit individual off-the-shelf chips before they are sealed into separate packages. In order to do this to do this it must be possible to test the operation of the circuit and its individual chips before final packaging. Testing should be under conditions as nearly as possible equal to actual operating conditions. This also means as a practical matter that one or more individual ICs in the overall circuit, in the event the circuit does not operate properly for a particular application, should be easily replaceable even though the individual IC has met general specifications for itself alone.
In the past there have been various attempts to provide electrical contacts which meet the needs outlined above. Most if not all such attempts have been not entirely successful because of one or more shortcomings such as: not sufficiently small in size; or without adequate robustness to make stable electrical connections directly to contact pads on the face of an IC, or without adequate compliance to repeatedly make contact to surface pads without permanent deformation or xe2x80x9csetxe2x80x9d, or without ease of manufacturing and of assembly with the necessary high degrees of mechanical precision and electrical performance.
As is well known in the art of electrical connections, a prime requirement for an electrical contact is that it be able to make a stable and low resistance connection. To do this a contact should itself have high conductivity and enough physical strength (i.e., be robust enough) to exert adequate force against the member it is contacting. A contact should also act against the member to break through or scrape away any oxide (or high resistance film) on the surface between contact and member. These requirements for stability and low resistance are extremely difficult to meet when it is also necessary that the size of contacts be minute enough to make simultaneous electrical connections to a thousand or so micron-size contact pads (e.g., 80 or so microns square) on an IC.
The present invention provides ultra-miniature electrical contacts which are strong yet compliant, together with an efficient method of manufacturing and of precision assembly into electrical connectors, especially suitable for stable, low resistance temporary and/or permanent connections to the closely spaced area array and/or edge array contact pods of very dense ICs. The present invention is an important improvement upon that described now U.S. Pat. No. 6,245,444 in U.S. Pat. No. 6,245,444, filed Oct. 2, 1997, in the names of Robert B. Marcus and Yanwei Zhang. The disclosure in said patent application is referred to herein and is explicitly incorporated by reference into the present application.
In accordance with one aspect of the invention there are provided ultra-miniature electrical contacts having improved performance and versatility. The design of one such contact insures that when it is pressed down against a surface pad on an IC for example, the contact wipes against the pad and breaks through or scrapes away oxide or other high resistance film on the face of the pad. The contact is of high-conductivity material and has ample strength and resilience to bear against the pad with enough force to insure stable, low resistance electrical contact Each contact is initially formed on a substrate as a thin, narrow elongated flat body comprised of selectively deposited layers of metal. Depending on the final configuration desired for the contact, as illustrated hereinafter, the metal of a first metal layer has a first coefficient of thermal expansion (xcex11), and the metal of another layer has a second thermal coefficient (xcex12), which is different from xcex11. Each contact is formed into a three-dimensional structure (as will be explained hereinafter) and is then made strong by a covering of a specialized metal plating which adds substantial stiffness and strength to the contact. This also makes the contact xe2x80x9cspringyxe2x80x9d (compliant) and enables the contact to be deformed substantially without permanent deformation or xe2x80x9csetxe2x80x9d, a characteristic highly important to multiple electrical contacts making connections to a large number of surface pads on a chip or to the surface pads on many chips in a planar array. This is especially so when making repeated connections to one IC after another, as with a probe card for example.
Electrical contacts according to the invention are advantageously fabricated en masse on closely spaced and precisely located centers by photolithography, and selective deposition of metals and other materials through a series of process steps akin to ones employed in semiconductor manufacturing and which are well known. By way of example, ultra-miniature electrical contacts in accordance with the present invention are produced by: (a) providing a substrate, such as a silicon wafer, and depositing a thin insulating layer thereon, such as silicon nitride (Si3N4); (b) depositing a thin sacrificial layer, such as SiO2; (c) patterning the sacrificial layer using photolithography, so that strips of the sacrificial layer left after patterning define, at least roughly, the outline in planar form of each such electrical contact; (d) defining photographically the regions for metal deposition with photo resist thicker than the thickness of the metal to be deposited; (e) depositing (e.g., by sputtering) a layer of metal having a first coefficient of thermal expansion; (f) removing the resist along with unwanted metal; (g) depositing and patterning photoresist for a second layer of metal; (h) selectively depositing (e.g., by sputtering) a layer of metal having a thermal coefficient of expansion different from the first; (i) removing the resist along with unwanted portions of metal; (j) removing the sacrificial layer (e.g., using hydroflouric acid); (k) heating the planar contacts to xe2x80x9ccurlxe2x80x9d or form them into three-dimensional shapes of desired configuration; and (1) substantially strengthening the contacts by electroplating over them specialized stiffening metal. In addition special contact areas may be applied to each electrical contact to provide a surface-scraping action when pressed against another contact member, such as a surface pad on an integrated circuit (IC).
In accordance with another apparatus aspect, the present invention is directed to an ultra-miniature electrical contact which has an elongated conductive body formed of at least two layers of different metals selectively deposited upon each other, a portion of the contact being adapted to be fixed to a substrate, another portion of the contact being permanently bent above the substrate as a result of differential expansion of metal layers, and an outer stiffening metal layer being deposited over the plurality of metal layers, such that the contact has sufficient strength and resilience to effect stable low-resistance connection to another contact member.
Viewed from another apparatus aspect, the present invention is directed to an ultra-miniature electrical contact comprising an elongated conductive body having two ends and formed of at least three layers of metals selectively deposited upon each other. The layers comprise an inner layer, a middle layer, and an outer layer with the middle layer being located between at least one portion of each of the inner and outer layers, the middle metal layer having a coefficient of thermal expansion that is different from that of other layers. One end of the body of the contact is adapted to be affixed to a substrate. The body of the contact is bent above the one end of the contact and the substrate. At least a portion of the body of the contact near its other end is bent down toward the substrate by differential expansion of metal layers.
Viewed from another apparatus aspect, the present invention is directed to an ultra-miniature electrical contact comprising an elongated conductive body formed of a plurality of two or more layers of different metals selectively deposited upon each other and an outer stiffening metal layer. A portion of the contact is adapted to be fixed to a substrate. Another portion of the contact is permanently bent above the substrate as a result of differential expansion of metal layers. An outer stiffening metal layer is deposited over the plurality of metal layers such that the contact has sufficient strength and resilience to effect stable low-resistance connection to another contact member.
Viewed from another apparatus aspect, the present invention is directed to an assembly of ultra-miniature electrical contacts mounted on a substrate and suitable for contacting contact areas of a device such as an integrated circuit. At least one of the ultra-miniature electrical contacts comprises an elongated conductive body having two ends and formed of at least three layers of metals selectively deposited upon each other. One inner metal layer has a coefficient of thermal expansion that is different from that of other layers. One end of the body of the contact is adapted to be affixed to the substrate, the body of the contact by differential expansion of metal layers being bent above the one fixed end of the contact and the substrate. At least a portion of the body of the contact near its other end is bent down toward the substrate as a result of differential expansion of metal layers. An outer stiffening metal layer is electrodeposited on the other layers to give substantial strength and resiliency to the contact.
Viewed from another apparatus aspect, the present invention is directed to an assembly of ultra-miniature electrical contacts mounted on a substrate and suitable for contacting surface pads of a device such as an integrated circuit (IC). The assembly comprises a substrate with an elongated conductive body formed on the substrate as a plurality of layers of different metals selectively deposited upon each other. A portion of the contact is affixed to the substrate. A remaining portion of the contact is permanently bent into a three-dimensional shape above the substrate as a result of differential expansion of metal layers. An outer stiffening metal layer is deposited over the plurality of metal layers such that the contact has sufficient strength and resilience to effect stable low-resistance connection to a surface pad of an IC device.
Viewed from a method aspect, the present invention is directed to a method of forming ultra-miniature electrical contacts. The method comprises the steps of: defining by photolithography an elongated area or areas on a substrate; selectively depositing within each area an inner layer, a middle layer, and an outer layer of metals deposited upon each other to form a conductive body of the contact with the middle metal layer having a coefficient of thermal expansion higher than that of other layers and one end of the contact being fixed to the substrate; and heating the body of the contact so that by differential expansion of the metal layers the contact for most of its length beyond its fixed end is bent above the substrate, and at least a portion of the body near its other end is bent down toward the substrate.
Viewed from another method aspect, the present invention is directed to a method of forming ultra-miniature electrical contacts. The method comprises the steps of: defining by photolithography an elongated area or areas on a substrate; selectively depositing within each area a plurality of layers of metal upon each other to form a conductive body of a contact with one metal layer having a coefficient of thermal expansion higher than that of another layer and one end of the contact being fixed to the substrate; heating the body of the contact so that by differential expansion of metal layers the contact for most of its length beyond its fixed end is bent above the substrate, and at least a portion of the body near its other end is bent down toward the substrate; and over-plating with an outer stiffening metal layer the other metal layers to give substantial strength and resiliency to the contact.
In accordance with yet another method aspect, the present invention is directed to a method of forming an assembly of ultra-miniature electrical contacts mounted on a substrate and suitable for contacting contact areas of a device such as surface pads of an integrated circuit (IC). The method comprises the steps of: providing an insulated substrate on which a pattern of conductive circuit traces can be deposited; defining by photolithography elongated areas on the substrate where contacts are to be located; depositing in each of the defined areas an elongated conductive body of a contact having a plurality of layers of different metals selectively plated upon each other, an end portion of one metal layer being connected to a portion of conductive traces on the substrate; heating each contact body so that by differential expansion of the metal layers the contact is permanently bent into a three-dimensional shape above the substrate; and depositing an outer stiffening metal layer over the plurality of metal layers so that the contacts have sufficient strength and resilience to effect stable low-resistance connection to contact areas of a device such as surface pads of an IC.
A better understanding of the invention together with a fuller appreciation of its many advantages will best be gained from a study of the following description given in connection with the accompanying drawing and claims.